Light is used by instruments to measure the flow or movement of gases, liquids and solids. These instruments include laser anemometers, blood perfusion monitors and systems that measure solid surface motion. The Doppler principle is the basic operating principle for each of these instruments. Light is focused on the moving material and is scattered by collisions which occur between photons of light and particles in the material. These collisions impart a frequency shift to the photons which is proportional to the velocity of the moving material. This frequency shift is difficult to measure directly, but by mixing the frequency-shifted light with light that has not been frequency-shifted, a slower beat frequency results. The beat frequency is also proportional to the velocity of the moving material and it can be measured. An example of this instrument is disclosed in U.S. Pat. No. 4,387,993. Flow measurements of this type require that the light have a single wavelength so these instruments usually use a laser or a laser diode as a light source. The light is focused at the measurement location by the focusing or transmitting optics. The receiving optics collect the scattered light and focus it onto the surface of a photo detector. The beat frequency results from the mixing of frequency-shifted light with light that has not been frequency-shifted on the surface of the photo detector. The photo detector converts this beat frequency to an electrical frequency which is then converted into a velocity or a flow measurement by electronic signal processing.
Optical fibers are used to replace all or part of the focusing and/or the receiving optics in these instruments. Incorporating fiber optics makes the instruments safer and easier to use and allows access to otherwise inaccessible measurement situations. A blood perfusion monitor, for example, both transmits light to and collects light from the measurement location with optical fibers. The measurement location in this case is the capillary bed just below the surface of a tissue, such as skin. The instrument non-invasively measures blood flow through the capillary bed. Using optical fibers in the blood perfusion monitor allows blood flow measurements to be made conveniently and safely because the fibers electrically isolate the patient from the instrument.
Whenever optical fibers are used to transmit light, an effective means must be used to connect the fibers to the rest of the system. The main requirement of this connection is that it provide good coupling of light from a light source into a fiber or from one fiber into another. In the case of flow-measuring instruments, the optical fiber that transmits light to the measurement location must have a small core diameter, on the order of 50 microns or less. Achieving good coupling of light from light source into fiber in these instruments is difficult. Once an instrument has been aligned, a change in location of the fiber end of even 10 microns can cause a substantial loss in the amount of light transmitted. Good alignment in the connection between the light source and the optical fiber can be achieved during the manufacturing process and the components can then be permanently fixed in place so that the connection cannot be broken. If a permanent connection between the light source and the optical fiber is not desirable, a connector having position adjustments can be used. Each time the optical fiber is connected to the light source, the user aligns the fiber so that a maximum amount of light is coupled into it. This type of connector requires that the user have the equipment and the skills necessary to make the alignment.
Optical fiber connector systems having quick connect and disconnect optical terminators without position adjustments are described by Cartier in U.S. Pat. No. 4,429,949 and McNaughton et al in U.S. Pat. No. 4,553,813. A rigid sleeve and a plurality of balls are used by Cartier to radially center optical fibers. McNaughton et al utilizes a clamp to hold an optical fiber connector on diverging ears to align optical fibers.